Avionics News November 2014 - 45

manager for the first half of his 33-year tenure, Ennenga
airframe is a challenge, but it's not insurmountable,
still participates in GA antenna design, and he's RAMI's
according to Ennenga. Certificated airplanes are designed
FAA-designated manufacturing inspection representative.
to dissipate lightning strikes and provide antenna ground
Antennas today "may look different, but they are still the
planes. Amateur builders of experimental aircraft have to
same operationally," he said.
be more creative. As he demonstrates every year at EAA
A communication antenna is a fiberglass rod that
AirVenture in Oshkosh, Wisconsin, the solution can be
encases a conductor. Like all antennas, the frequencies
four 22-24-inch lengths of 16 to 18 gauge wire connected
transmitted and/or received determine its length.
to the antenna mount. Situated every 90 degrees, the
Physically, there's only so much engineers can do to
four ground plane wires will fill the void between them
accommodate aerodynamic and aesthetic desires of
electrically. Finding the necessary space fore and aft
aircraft manufacturers and owners. "You can make it
usually isn't a problem, he said, but most fuselages are
smaller to reduce its visual signature, but when you
not 44 to 48 inches wide. Running the ground plane wires
reduce an antenna's size you
down the fuselage sides works in
compromise its performance,"
the same way as the skin of a metal
Ennenga said.
airplane.
THE MATERIALS THAT CREATE
The intimate relationship
Battered by the slipstream
ANTENNAS HAVE CHANGED
between an antenna and its
and baked by the sun, antennas
- MODERN COMPOSITE
frequency is why no single antenna
have a hard life. "Many antennas
MATERIALS HAVE REPLACED
can cover the entire aviation
incorporate matching networks
WOODEN POLES -
spectrum, which starts at 190using capacitive inductive circuits,
1,750 kHz with the ADF and ends
and those don't often fail," Ennenga
at 1,575.42 MHz with GPS. Because
said. "Most failures are caused by
their frequencies are neighbors, the
physical damage." Erosion of the
VOR/LOC (localizer) and glideslope
composite structure will not affect
can share a blade-style antenna;
performance, "unless the antenna is
a splitter box divides the two
starting to suck in moisture; this has
into separate connections to the
more of an effect on sharply tuned
navigational receiver. Newer nav/comms have integrated
antennas, like a GPS or marker beacon, than it does on a
splitters and a single nav antenna connection. This can
broadband antenna, like a comm." Before moisture does
pose a problem in a dual nav/comm stack when replacing
its damage, erosion can be repaired with epoxy resin
just one radio, according to Ennenga. RAMI solved this
and good quality polyurethane paint that does not have
connection disparity with a splitter diplexer.
any metal in it. He does not recommend paint for GPS
What's mounted to the airplane is only half of an
and marker beacon antennas.
antenna system. Its effectiveness depends on a ground
plane, an electrical connection to a conductive surface
Location, location, location
that reflects the radio waves. This surface must surround
The biggest influence of avionics technology on antennas
the antenna for a quarter of its wavelength, which for a
is the number that must find a home on today's aircraft.
comm antenna is 22 to 24 inches. On metal airplanes, this
ADS-B is the most recent addition. Depending on the
isn't a problem because the entire airframe is a ground
aircraft's operational environment, it may need an antenna
plane. As it is with any good electrical connection, a
above and below the fuselage.
good bond between the antenna and its ground plane
New avionics systems, such as GPS, work with extremely
is critical. Some antennas make this connection through
low signal strengths that present "interesting challenges,"
the base on bare metal skin, said Ennenga, "but we
said Bill Stone, Garmin's senior manager of business
create this electrical bond with the antenna's mounting
development. "The tiny, tiny signal that reaches Earth
hardware."
Continued on following page
Creating a ground plane on a composite or wood

BUT THE
FUNDAMENTAL
DESIGN REMAINS
THE SAME.

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Avionics News November 2014

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